I'm no computer guru but I can tell you that I recently went from Vista 32 bit to Windows7 64 bit and have gotten much better performance. How that would compare to XP, I"m not sure.

As for my computer, I only have 4GB of ram and made no change with the upgrade, though I did go from multiple symlinked folders with Keyclone to one folder with ISBoxer. Additionally, I did not do an upgrade, I saved the things I wanted off my computer, wiped it, and did a clean install of Windows7.

For comparisons sake, I used to have to walk in Dalaran while boxing and would still have toons break follow whereas now I can easily mount all 5 of them, put on the pally aura and run all over the place without losing toons.

Hope that helps.